A reflector array (or “reflectarray”) antenna 10, as represented for example in FIG. 1, comprises a set of individual radiating elements 12 assembled in a one- or two-dimensional array 11 and forming a reflecting surface 14 making it possible to increase the directivity and the gain of the antenna 10. The individual radiating elements, also called individual cells, of the reflector array, of metal patch and/or slot type, have variable parameters, such as, for example, the geometric dimensions of the etched patterns (length and width of the patches or of the slots) which are set so as to obtain a selected radiation pattern. As shown for example in FIG. 2, the individual radiating elements 12 can consist of metal patches filled with radiating slots and separated from a metal ground plane by a typical distance of between λg/10 and λg/6, in which λg is the guided wavelength in the spacing medium. This spacing medium may be a dielectric, but also a composite sandwich produced by a symmetrical lay-out of a bees nest type separator and of dielectric skins of fine thicknesses. For the antenna 10 to be efficient, it is essential for the individual cell to be able to accurately control the phase shift that it produces on an incident wave, for the different frequencies of the bandwidth. The method of manufacturing the reflector array must also be as simple as possible.
The lay-out of the radiating elements in the reflector array requires great attention. It should observe, at least approximately, a strong periodicity which defines the reflection characteristics of the reflector array (typically less than 0.65λ and preferably equal to 0.5λ, in which λ is the wavelength in free space). As explained below, the greater the periodicity, the better the efficiency. However, the reflector arrays that are currently known present a major problem.
The lay-out of the individual radiating elements relative to one another to form a reflector array is synthesized so as to obtain a given radiation pattern in a pointing direction that is selected to produce a given coverage. FIG. 3a shows an exemplary arrangement of the radiating elements of a reflector array antenna according to the prior art, making it possible to obtain a directional beam pointed in a lateral direction relative to the antenna. Because of the flatness of the reflector array and because of the differences in the path lengths of a wave emitted by a primary feed 13 to each radiating element of the array, the illumination of the reflector array by an incident wave originating from a primary feed 13 produces a phase distribution of the electromagnetic field above the reflecting surface. The dimensions of the radiating elements are therefore defined so that the incident wave is reflected by the array 11 with a phase shift that compensates the relative phase of the incident wave. The radiating elements 12 are therefore not all surrounded by similar elements, and the transitions from one radiating element to another are greater the faster the phase variation.
As a result of this, there are two problems: firstly, the standard approximation which consists in calculating the electrical characteristics of the radiating elements assuming an infinite periodicity is no longer valid for these elements. Also, a diffraction phenomenon appears in these areas where the pseudo-periodicity of the arrangement of the individual radiating elements 12 is broken. Although the amplitude of the electric field is assumed to follow an apodized distribution related to the width of the beam from the primary feed 13, the measured distribution of the radiated electric field above the reflector array 11 as a whole exhibits areas in which it is damped, which correspond precisely to the location of these strong transitions. The greater the mesh size of the reflector array, the greater this diffraction becomes. This causes an increase in the level of the secondary lobes which, even it remains less than −20 dB, creates a degradation of the directivity of the associated antenna 10 which is unacceptable for a telecommunication antenna.